If you were to get 100 yes or no answers, what benefit would you get from those answers?

Let me say it so. If someone would say "no" I as an Admin, Mod,... would delete that post. If someone say Yey I would ask which Bridge he use.

are you looking for a recommendation for a bridge?

Sure. There is nobody who know anything beside of God and Google.

are you just curious?

...
The point is in an other Forum I asked about a "Tile Server" and nobody known about so I saved a lot of time to Translate (in my Mind) my Question.
So when someone (here) know something about an special stuff its easy to replay with the full storry.
How does the World would be today if none had cared what Mr. Hitler sayed in the Bavarian Restaurant Basement.

So its not a secret hat I my Hoster (and I) use the C-Panel. There my Forum and (later) some Homepages (with Joomla) will run.
The Support from the Bridge Company told me I cant use "p8pBB comments" with Multible Domains. The question is why?
Everything run in the same C-Panel Account. So what does the use as Bridge? The DB? If yes that should not be a problem to Create one DB where every Installation can read/ write into.
Or the File System? I guess there is some way to write Data on the Server outside of the Domain Name.
Or its just something who nobody thought about and no one will write something for?!

if you want to know if there is a bridge for joomla you ask if there is a bridge for joomla, not if anyone uses one.

as for the rest of your post, it really didn't make much sense.

cpanel has nothing at all to do with this.

a phpbb/joomla bridge would simply be a script that will allow joomla and phpbb to share users. that way people would not have to login to each one separately.
I have no idea what this means:

The Support from the Bridge Company told me I cant use "p8pBB comments" with Multible Domains.

you don't normally have two different "domains" for something like this and there is no such thing as "phpbb comments" in phpbb.

you would have something like: phpbb.yourdomain.com for phpbb and joomla.yourdomain.com for joomla
or, yourdomain.com/phpbb and yourdomain.com/joomla
then you would install your bridge script that makes one signup and one login for both.